1. Field of the Invention
The present invention concerns a damping apparatus that particularly but not exclusively is designed for boring bars and similar tool holding bars used in machine tools. Typical examples are boring bars used for chip cutting machining (turning, milling, etc.) on weak constructions, such as shafts and similar elements. The apparatus may also be used on “clampings”, other parts of a machine, and works pieces. Self generating oscillations are typically a result of the dynamic rigidity of a construction being too small.
2. Description of the Related Art
Similar damping apparatuses are previously known, and a related apparatus is shown in NO patent 128 725. This patent concerns a damping device that necessarily must be placed in a boring in the boring bar, because the boring itself constitutes an important part of the damping device, in that the damping effect is a result of an interaction between the damping body and the boring itself, through resilient elements provided between the damping body and the boring.
The development evolves towards an increased ration between length/diameter (overhang) on tool holders. This results in a reduction of the dynamic rigidity of the tool holder and this again impose new requirements to the damping device that must be adapted accordingly. The problem with the damping device according to the known art is that to adapt the damping device to these new conditions, the frequency of the damping body must be lowered. To achieve this, either resilient elements with a lower spring constant, or a damping body with a larger mass is required.
The density of the damping bodies used to day is already among the highest commercially available. To increase the mass, the volume of the damping body must therefore be increased. This will result in problems related to space and size and weakening of the construction where the damping apparatus is to be placed.
To be able to reduce the spring constant with solutions according to the known art, as for instance described in NO patent 172 677, must accordingly either the material in the resilient element be changed to a softer material with a lower spring constant, or the strength of the resilient element must be reduced by reducing the size of the element. At the same time, the resilient elements must resist heat that is developed during the chip cutting process, and as a result of energy absorption in the material, in addition to being dimensioned to withstand forces they are exposed to. This has proved to be contradicting properties.
The conclusion is that the solutions according to the known art therefore not is well suited in the development towards an increased length/diameter ratio and a lower natural frequency for a system that is to be damped, due to limitations in material properties amongst other things.
High rotational speeds are often used, and it is therefore also important that a damping system can withstand fast rotation. This has also proved problematic with apparatuses according to the known art.